PhyslibAlpha.AlgebraicFramework.OrderUnit.Symmetry
Symmetry: order-automorphisms and their action on states
i. Overview
Physically, a symmetry of a system is a *reversible* transformation: a change of description that loses no information and can always be undone. In the order-unit picture (`Channel/Basic.lean`) a transformation between effect algebras is a channel, a unital positive linear map; a symmetry is exactly a channel `φ : E →ₚ₁[ℝ] E` that is invertible with an inverse `φ⁻¹` that is *also* a channel. Both directions have to be physical: undoing a symmetry must again be positive and send the certain event to the certain event, not just be some linear inverse.
A group `G` acting on `E` "by symmetries" is then a homomorphism `ρ : G →* Symmetry E` into the group of such automorphisms. Every symmetry, via `UnitalPositiveLinearMap.comp`, transports a state: precomposing a state `ω : E →ₚ₁[ℝ] ℝ` with `φ⁻¹` gives the state that assigns to an observable `x` whatever `ω` assigned to the pulled-back observable `φ⁻¹ x`. This needs no new proof of positivity or normalization — `ω.comp φ⁻¹` is already a unital positive linear map because `.comp` always is; all that is new here is checking this assignment is a genuine group action, `(g*h) • ω = g • (h • ω)` and `1 • ω = ω`, which reduces to associativity and identity laws for `.comp` already on hand.
This is the state-level pushforward along a channel, specialized to a channel that happens to be invertible (`Channel/Basic.lean`'s `Weight.comp` is the same idea one level down, on weights on the positive cone; working with the state directly as a `UnitalPositiveLinearMap` avoids the extra order-unit hypotheses `Weight.comp` needs and is the cleaner route here).
ii. Key definitions and results
- `IsOrderAutomorphism φ`: `φ` is a channel with a two-sided inverse that is also a channel. - `Symmetry E`: the bundled group of order-automorphisms of `E`, with `Group` instance `mul := comp`. - The `MulAction (Symmetry E) (𝓢[ℝ, E])` instance: pushing a state forward along the inverse automorphism. - `stateSMul`: the action of a general `G` on `𝓢[ℝ, E]` induced by a homomorphism `ρ : G →* Symmetry E`, together with the group action laws it satisfies. - `OneParameterAutomorphismGroup E`: a one-parameter (reversible dynamics) family `α : ℝ → (E →ₚ₁[ℝ] E)` with `α 0 = id` and `α (s + t) = α s ∘ α t`, each `α t` an automorphism.
iii. Table of contents
- A. Order automorphisms
- B. The symmetry group
- C. The induced action on states
- D. One-parameter automorphism groups
A. Order automorphisms
B. The symmetry group
C. The induced action on states
D. One-parameter automorphism groups: reversible dynamics
33 declarations
is an order-automorphism
A unital positive linear map (also called a channel) is an order-automorphism if there exists another unital positive linear map such that and . This implies that is a reversible transformation where both the map and its inverse preserve the order and the unit of the space .
The identity map is an order-automorphism
The identity map is an order-automorphism.
Inverse of an order-automorphism
Given a unital positive linear map (also referred to as a channel) and a proof that is an order-automorphism, this definition provides the inverse unital positive linear map . This inverse map satisfies the conditions and , ensuring that the reversal of the transformation is itself a valid channel (i.e., it preserves both the order and the unit of the space ).
for an order-automorphism
Let be a unital positive linear map that is an order-automorphism, and let be its inverse. Then the composition of and is the identity map on , written as .
for an Order-Automorphism
Let be a space and be a unital positive linear map (a channel). If is an order-automorphism, let denote its inverse unital positive linear map. Then the composition of with its inverse is the identity map on , denoted as .
for an order-automorphism
Let be a unital positive linear map (a channel) on an order-unit space . If is an order-automorphism with inverse , then for any , applying followed by returns the original element:
for order-automorphisms
Let be an order-automorphism (a unital positive linear map with a unital positive linear inverse ). For any element , applying the inverse map followed by the map returns the original element:
The inverse of an order-automorphism is an order-automorphism
Let be an order-unit space and be a unital positive linear map. If is an order-automorphism, then its inverse is also an order-automorphism.
The composition of two order-automorphisms is an order-automorphism
Let be a space and be unital positive linear maps (channels). If and are order-automorphisms, then their composition is also an order-automorphism.
An order-automorphism is bijective
Let be an order-unit space (a real ordered vector space with a distinguished unit element) and let be a unital positive linear map. If is an order-automorphism, then is a bijective function.
Symmetry of
Let be a real vector space equipped with a partial order and a unit element . The type `Symmetry E` is the set of all unital positive linear maps (also known as channels) that are order-automorphisms. A map is an order-automorphism if there exists an inverse unital positive linear map such that and . This structure bundles the map with the proof that it and its inverse preserve both the ordering and the unit of .
Extensionality of symmetries:
Let be a real vector space equipped with a partial order and a unit element. If and are symmetries of (unital positive linear maps that are order-automorphisms) such that holds for every , then .
Identity element of
The identity element for the group of symmetries is defined as the identity unital positive linear map .
Multiplication in as composition
Given two symmetries of a real ordered vector space with unit , their multiplication is defined as the composition of the underlying unital positive linear maps . This composition results in a new order-automorphism of .
Inverse of a symmetry
This definition provides the inverse operation for the group of symmetries of . For any symmetry , its inverse is the symmetry whose underlying map is the inverse unital positive linear map (channel) satisfying and .
The identity of is
Let be a real ordered vector space with unit. The underlying unital positive linear map of the identity element in the group of symmetries is the identity map on .
The underlying map of is
Let be a real ordered vector space with a unit. For any two symmetries , the underlying unital positive linear map (channel) of their product in the symmetry group is equal to the composition of their individual underlying maps.
The underlying map of the inverse symmetry is the inverse of the map of
Let be a real vector space equipped with a partial order and a unit element. For any symmetry , the underlying unital positive linear map (channel) of the inverse symmetry is equal to the inverse channel of .
Symmetry group of
The set of order-automorphisms of a real ordered vector space with unit element forms a group. In this group, the multiplication of two symmetries and is defined as their composition , the identity element is the identity map , and the inverse of a symmetry is its inverse order-automorphism .
Group isomorphism
For a real ordered vector space with unit , there is a group isomorphism between the group of symmetries and the group of units of the monoid of unital positive linear maps (channels) on . This equivalence maps each symmetry to the unit whose underlying value is the channel and whose inverse is the channel .
Group action of on the state space via
The group of symmetries acts on the state space (denoted as ). For a symmetry and a state , the action is defined by , where is the inverse order-automorphism. This corresponds to the Schrödinger-picture pushforward of a state, where the state is transported by pulling it back along the inverse of the symmetry transformation.
Action of symmetry on state is
Let be a real ordered vector space with a unit element. For any symmetry and any state , the group action of on is defined by the composition , where is the inverse order-automorphism of .
Group action of on the state space induced by
Let be a group and be a real ordered vector space with a unit element. Given a group homomorphism that represents as a group of order-automorphisms of , the function `stateSMul` defines an action of on the state space . For any element and state , the action results in a new state given by . Following the definition of symmetry actions on states, this is equivalent to the composition .
Let be a group and be a real ordered vector space with a unit element. For any group homomorphism and any state , the action of the identity element on the state satisfies: where denotes the group action of on the state space induced by .
for the induced action on states
Let be a group and be a real ordered vector space with a unit element. Let be a group homomorphism that represents as a group of order-automorphisms of . For any elements and any state , the action of the group on the state space satisfies the property , where the action is defined by `stateSMul ρ g ω`.
Coercion of `OneParameterAutomorphismGroup` to
This instance defines a coercion from the type of one-parameter automorphism groups `OneParameterAutomorphismGroup E` to a function type. It allows an element representing a one-parameter family of symmetries to be treated as a mapping that takes a real number and returns a unital positive linear map (channel) .
for one-parameter automorphism groups
For any one-parameter automorphism group on a system , the transformation corresponding to the parameter is the identity map on , denoted as .
for one-parameter automorphism groups
Let be a system (represented as an order-unit space or effect algebra) and let be a one-parameter automorphism group on . For any real numbers and , the automorphism associated with the sum is the composition of the automorphisms associated with and :
is an Order-Automorphism with Inverse
Given a one-parameter automorphism group on a space (such as an effect algebra or order-unit space), for every , the map is an order-automorphism. This means is a unital positive linear map with a two-sided inverse that is also a unital positive linear map. Specifically, the inverse of is given by , representing the physical intuition that running time backwards undoes the transformation.
One-parameter automorphism group as a group homomorphism
Let be a real ordered vector space with a unit element. A one-parameter automorphism group on is organized into a group homomorphism from the additive group of real numbers (represented as `Multiplicative ℝ`) to the group of symmetries . This map associates each with the order-automorphism , satisfying and . This formulation allows the dynamics to be treated as a general group action on the state space of the system.
State evolution induced by a one-parameter automorphism group
Let be a real ordered vector space with a unit element, and let denote its state space (the set of unital positive linear maps from to ). Given a one-parameter automorphism group on and an initial state , the function `stateEvolution` defines the state at time . It is calculated as the action of the symmetry on the state , which is equivalent to the precomposition . This evolution satisfies the group action properties: and .
Let be a real ordered vector space with a unit element, and let be its state space. For any one-parameter automorphism group on and any initial state , the state evolution at time is the identity, such that .
Let be a real ordered vector space with a unit element, and let denote its state space (the set of unital positive linear maps from to ). For any one-parameter automorphism group on , any initial state , and any time parameters , the state evolution satisfies the group property:
